PT Friction & Wobble Loss Calculator

Example Data Table

Formula Used

The tendon force at distance x from a stressing end is computed using the standard friction-and-wobble expression:

• P0 is the jacking force at the active end.
• μ is the friction coefficient between strand and duct.
• θ(x) is the cumulative angular change to point x (radians).
• k is the wobble coefficient (per unit length).

For two-end jacking, this tool computes the force profile from each end and combines them using either an envelope (conservative) or average (balanced) assumption.

How to Use This Calculator

1. Choose metric (m) or imperial (ft) units consistently.
2. Enter the jacking force and select one-end or two-end.
3. Set realistic values for μ and k from your specs.
4. Split the tendon path into segments with lengths and angle changes.
5. Press Calculate Losses to see force and loss profiles.
6. Download CSV for records or PDF for site reporting.

Always verify assumptions with project specifications and stressing logs.

PT Friction and Wobble Loss in Construction Practice

1) Why friction and wobble matter on site

Post-tensioning force reduces as the strand travels through the duct. Two mechanisms dominate: curvature friction (controlled by μ and total angular change θ) and unintended alignment deviation, called wobble (controlled by k and length x). On long tendons, wobble often becomes the larger share of loss, especially when chair spacing is wide.

2) Typical parameter ranges used for checks

Field checks commonly use μ between 0.15–0.30, depending on the system, duct type, cleanliness, and lubrication. Wobble k frequently falls around 0.001–0.003 per meter (or an equivalent per-foot value). Higher k is often linked to duct waviness, tight tolerances, or congested reinforcement zones.

3) Segmenting the tendon for realistic geometry

Instead of a single “total length,” this calculator breaks the tendon path into up to six segments. Each segment stores two measurable inputs: length and angle change in degrees. The tool converts degrees to radians and accumulates θ to each endpoint, producing a force profile that is easy to reconcile with shop drawings and as-built measurements.

4) One-end versus two-end jacking options

One-end jacking uses a single stressing force P0 and reports the decay along the full length. Two-end jacking calculates decay from both ends and combines them. The Envelope method takes the higher force at each point (conservative when checking minimum force), while Average provides a balanced estimate when stressing is near-symmetric and both ends reach similar forces.

5) What the results table tells you

The profile table reports cumulative distance x, cumulative θ, and the computed forces from each end. It also shows loss percentage relative to the reference jacking force. If you enter tendon area, the calculator reports stress in MPa, helping crews compare against allowable stressing limits and shop drawings.

6) Quality-control checks using project data

Compare the calculated right-end force with stressing records and elongation-based back-calculations. Large gaps may indicate incorrect μ/k assumptions, unexpected curvature, duct damage, poor alignment, or seating losses not modeled here. As a practical screen, flag cases where right-end loss exceeds 10–15% on moderate lengths, and review the geometry and coefficients before proceeding.

7) Documentation and reporting benefits

Site teams often need quick documentation for approvals. The CSV export supports logbooks and trending, while the PDF export is useful for daily reports and submittals. Record the tendon ID, stressing sequence, achieved forces, and the μ/k basis (specification value, supplier data, or calibration note). This improves traceability during audits.

Always coordinate final stressing assumptions with project specifications and the PT supplier’s procedures.

Frequently Asked Questions

1) What units should I use for k?

k must match the length unit. If you use meters, enter k per meter. If you switch to feet, convert k to per foot. Keeping μ dimensionless and k consistent prevents incorrect exponential decay.

2) How do I estimate the angle change for a draped tendon?

Use the tendon profile geometry from drawings or survey. Sum curve segments as total angular change, not slope. If you know radius R and arc length s, angle in radians is approximately s/R.

3) Why does a long straight tendon still lose force?

Wobble loss applies even when θ is near zero. Small alignment deviations, duct waviness, and support spacing create an effective k·x term that reduces force with distance.

4) Which two-end method should I select?

Use Envelope for conservative minimum-force checks or staged stressing. Use Average for near-symmetric stressing where both ends reach similar forces. Confirm the method with your engineer’s assumptions.

5) Does this include anchorage seating loss?

No. This calculator focuses on friction and wobble. Seating loss reduces force near anchorages and is typically modeled separately using wedge seating values and tendon stiffness.

6) Can I model multiple curvature zones accurately?

Yes. Break the path into segments and assign an angle change for each curvature zone. Straight portions should have angle change set to zero. The tool accumulates θ endpoint-by-endpoint.

7) What inputs should I save in the project record?

Save tendon ID, jacking forces, μ, k, segment lengths, segment angle changes, and the chosen two-end method. Attach stressing logs and elongation checks alongside the exported CSV or PDF.